gdb: fix shellcheck warnings SC2154 (referenced but not assigned) in gdbarch.sh

gdb: fix shellcheck warnings SC2154 (referenced but not assigned) in gdbarch.sh

Fix all instances of this kind of warning:

    In gdbarch.sh line 96:
                    m ) staticdefault="${predefault}" ;;
                                       ^-----------^ SC2154: predefault is referenced but not assigned.

These warnings appear because we are doing something a bit funky when reading
the gdbarch fields.  These variables are not assigned explicitly, but
using some `eval` commands.

I don't think there is so much we can fix about those warnings.  To
silence them, I've changed `${foo}` to `${foo:-}`.  This tells the shell
to substitute with an empty string if `foo` is not defined.  This
retains the current behavior, but the warnings go away.

gdb/ChangeLog:

	* gdbarch.sh: Use ${foo:-} where shellcheck would report a
	"referenced but not assigned" warning.